In the wake of the Sept. 11 suicide hijackings, security has been tightened aboard the giant vessels that can stretch nearly a quarter of a mile and carry thousands of passengers.

In the United States, Coast Guard boats have been escorting cruise ships into port since the attacks on the World Trade Center and the Pentagon, and cruise companies have been submitting crew and passengers' names to the FBI and immigration officials for checks.

"We have always had security measures in place," said Tim Gallagher, spokesman for Carnival Cruises. "But since the attacks, we've gone to level three security, the highest security level there is."

Still, security experts say that with attention focused on air safety, cruise ships could be enticing targets for terrorists.

"When you protect air, land and other targets, terrorists are going to look for soft targets," said Rohan Gunaratna, a research fellow at the Center for Study of Terrorism and Political Violence at Scotland's University of St. Andrews. "Cruise ships are considered prestigious because there is a perception that they are filled with wealthy Americans."

Gunaratna, who has been asked by various governments to work as a consultant and question terrorists from the Middle East, Latin America and Asia, said groups such as Hamas, Hezbollah and al-Qaida are being trained for maritime attacks.

In Sri Lanka, divers have planted explosives on commercial ships and suicide bombers have sunk navy vessels. In the South China Sea, pirates have attacked commercial vessels. And in Yemen, suicide bombers attacked the destroyer USS Cole in October 2000, killing 17 U.S. sailors.

Cruise ship officials say that since Sept. 11 they have added security personnel and increased staff, making their ships far less vulnerable than planes. They also point to the industry's safety record only one large cruise ship has been hijacked since 1985 and say modern construction with watertight compartments makes ships difficult to sink.

Kim Petersen, chief executive officer of SeaSecure LLC, a maritime security consultant in Fort Lauderdale, Fla., said armed passengers would be hard-pressed to get aboard a ship, and if terrorists bombed a hull, casualties would be low.

"The cruise industry has, for years, had active anti-terrorism programs in place to prevent an array of potential attacks," said Petersen. "Those programs, coupled with stringent security, make taking a cruise one of the safest options Americans have."

Governments forced the industry to take up minimum security measures after terrorists hijacked the Italian cruiseliner Achille Lauro in 1985 and killed an American passenger.

Gunaratna said that attack had sweeping ramifications. "Only one American died aboard the Achille Lauro yet it took years for the industry to recover," he said. "It would be unlikely that terrorists would inflict heavy damage on a cruise ship but even if they managed to kill 10 to 15 people, it would have a huge impact and people, especially Americans, would stop taking cruises."